The objective of the proposed research is to extend the acoustic theory of speech production to include the influences of subglottal resonances. The simplified source-filter theory will be refined to include the subglottal system and the influences of both the sub- and supraglottal systems on the voice source. An acoustic model based on this theory will be implemented on a computer. Experimental data from a number of speakers will be obtained and acoustic analysis of this data will be performed to verify and support the new model. Results of the acoustic analysis will be compared across gender and an attempt will be made to explain any differences in terms of anatomical differences. An understanding of subglottal coupling through acoustic modeling and analysis will be beneficial to studies of glottal characteristics, disordered speech, and speech synthesis and analysis systems. The potential influence of subglottal resonances in shaping the vowel categories in language will also be examined with a combination of acoustic analysis and perceptual tests.